可以根据条件来更新指定的文档，但要注意查询条件的唯一性：
> db.tianyc04.remove()
> db.tianyc04.insert({name:'tyc',age:20})
> db.tianyc04.insert({name:'tyc',age:30})
> db.tianyc04.insert({name:'tyc',age:32})
> db.tianyc04.find()
{ "_id" : ObjectId("510c83ad13d7e50c2281b048"), "name" : "tyc", "age" : 20 }
{ "_id" : ObjectId("510c83b113d7e50c2281b049"), "name" : "tyc", "age" : 30 }
{ "_id" : ObjectId("510c83b313d7e50c2281b04a"), "name" : "tyc", "age" : 32 }
> x=db.tianyc04.findOne()
{ "_id" : ObjectId("510c83ad13d7e50c2281b048"), "name" : "tyc", "age" : 20 }
> x.age++
20
> x
{ "_id" : ObjectId("510c83ad13d7e50c2281b048"), "name" : "tyc", "age" : 21 }
#按照name条件更新，将会更新所有记录。但x中已经包含了_id字段，这样会产生主键重复，所以mongo只会更新第一个。
> db.tianyc04.update({name:'tyc'}, x)
> db.tianyc04.find()
{ "_id" : ObjectId("510c83ad13d7e50c2281b048"), "name" : "tyc", "age" : 21 }
{ "_id" : ObjectId("510c83b113d7e50c2281b049"), "name" : "tyc", "age" : 30 }
{ "_id" : ObjectId("510c83b313d7e50c2281b04a"), "name" : "tyc", "age" : 32 }
#如果我更新第二条数据，会怎么样呢：
> x=db.tianyc04.findOne({age:30})
{ "_id" : ObjectId("510c83b113d7e50c2281b049"), "name" : "tyc", "age" : 30 }
> x.age++
30
> x
{ "_id" : ObjectId("510c83b113d7e50c2281b049"), "name" : "tyc", "age" : 31 }
> db.tianyc04.update({name:'tyc'},x)
> db.tianyc04.find()
{ "_id" : ObjectId("510c83ad13d7e50c2281b048"), "name" : "tyc", "age" : 21 }
{ "_id" : ObjectId("510c83b113d7e50c2281b049"), "name" : "tyc", "age" : 30 }
{ "_id" : ObjectId("510c83b313d7e50c2281b04a"), "name" : "tyc", "age" : 32 }
> db.getLastError()
null
#可以看到第二条数据并没有被更新掉。因为mongo会首先匹配到第一条记录，尝试将其更新为x，但此时会与第二条记录产生主键重复，所以mongo会直接退出，且没有提示错误。
#下面我按照_id来更新第二条记录，这次就更新正常了：
> db.tianyc04.update({_id:ObjectId("510c83b113d7e50c2281b049")},x)
> db.tianyc04.find()
{ "_id" : ObjectId("510c83ad13d7e50c2281b048"), "name" : "tyc", "age" : 21 }
{ "_id" : ObjectId("510c83b113d7e50c2281b049"), "name" : "tyc", "age" : 31 }
{ "_id" : ObjectId("510c83b313d7e50c2281b04a"), "name" : "tyc", "age" : 32 }
>
下面再无聊地使用tcy条件连续更新一下，就看到mongo的更新流程了：
> db.tianyc04.update({name:'tyc'},{name:'new_line'})
> db.tianyc04.find()
{ "_id" : ObjectId("510c83ad13d7e50c2281b048"), "name" : "new_line" }
{ "_id" : ObjectId("510c83b113d7e50c2281b049"), "name" : "tyc", "age" : 31 }
{ "_id" : ObjectId("510c83b313d7e50c2281b04a"), "name" : "tyc", "age" : 32 }
> db.tianyc04.update({name:'tyc'},{xjbu:'bengcaca'})
> db.tianyc04.find()
{ "_id" : ObjectId("510c83ad13d7e50c2281b048"), "name" : "new_line" }
{ "_id" : ObjectId("510c83b113d7e50c2281b049"), "xjbu" : "bengcaca" }
{ "_id" : ObjectId("510c83b313d7e50c2281b04a"), "name" : "tyc", "age" : 32 }
> db.tianyc04.update({name:'tyc'},{tonight:'papapa'})
> db.tianyc04.find()
{ "_id" : ObjectId("510c83ad13d7e50c2281b048"), "name" : "new_line" }
{ "_id" : ObjectId("510c83b113d7e50c2281b049"), "xjbu" : "bengcaca" }
{ "_id" : ObjectId("510c83b313d7e50c2281b04a"), "tonight" : "papapa" }
>
总结：更新时的查询条件建议选择_id。